Preparation of microporous materials with application in catalysis.
This invention concerns a swellable mixed laminar oxide capable of being delaminated into individual structural layers with microporous canals and cavities.
Laminar materials such as clays, zirconium phosphates and phosphonates, hydroxycarbonates of the hydrotalcite type, silicic acids (kanemite, magadite, etc.), transition metal sulphides, graphite, laminar hydroxides and others, can become swollen in the presence of water and/or appropriate interlaminar cations. The individual laminae of these materials are maintained together through weak bonds of the hydrogen bond type and/or electrostatic interactions. These bonds are easily broken when the intercalation force or solvation energy of the cations is greater than the interlaminar attraction forces. This is the case, for example, with sodium montmorillonite which swells up to interlaminar distances of over 10 nm, in the presence of an excess of water. The interesting aspect of the swollen materials is to make the interlaminar space accessible to the reactive molecules and, consequently, the internal surface, considerably increasing the accessible active surface of the catalyst. When the material intercalated between the mixed oxide laminae is eliminated by calcination, the swollen laminar compound collapses, recovering the original interlaminar distance.
Several procedures have been developed in order to avoid the interlaminar collapse. One of these consists of the intercalation, by interlaminar exchange or solvation of cations, of polar molecules with very long hydrocarbon chains which give rise to materials with very large separation distances between laminae. In these conditions, the interlaminar attraction forces are very weak, and a subsequent treatment, for example, with ultrasounds or stirring, can lead to definitively separate the laminae from each other.
Spanish patent ES-2124154 refers to a preparation method of a microporous solid with catalytic properties and with high external surface, called ITQ-2, which is synthesized through an intercalation procedure of organic molecules between the laminae of the appropriate precursor and subsequent delamination. However the properties of this solid as catalyst or catalyst additive need to be improved for the purposes of specific industrial processes.
The present invention includes the method of preparation of the laminar solid and its subsequent treatment to the point of achieving a highly-accessible microporous material with acidic characteristics capable of being used as a catalyst.
In the present invention, a material is obtained which we have called ITQ-18, with a microporous structure and extensive external surface, capable of supporting Brxc3x6nsted and Lewis acid centers. This new material is obtained from a swollen laminar precursor which we shall call PREITQ-18, transforming it through subsequent treatments into a delaminated material. The ITQ-18 material obtained in this invention, and which is characterised by its X-ray diffractogram, has very specific properties when it is used as a catalyst in reactions with organic compounds.
The present invention also refers to a preparation process of ITQ-18 which comprises:
an initial stage of obtention of a laminar material wherein 4,4xe2x80x2-bipyridyl as organic compound is used,
the swelling of said laminar material through the intercalation of organic molecules which have a proton-acceptor group and a hydrocarbon chain between the laminae of said laminar material, wherein PREITQ-18 is obtained,
the at least partially delamination of PREITQ-18 by using mechanical stirring, ultrasounds, spray-drying, lyophilization or a combination thereof and
calcination.
The preparation process therefore consists of, in an initial stage, the synthesis of a swollen laminar precursor (PREITQ-18) which is obtained by mixing a source of silicon in an autoclave, such as Aerosil, Ludox, tetraethyl orthosilicate (TEOS), sodium silicate or any other known sources, a source of aluminium, such as Al2(SO4)3, AlCl3, boehmite, pseudoboehmite, or any other, a source of sulphates, such as sulphuric acid and an organic compound which is 4,4xe2x80x2-bipyridyl, in addition to water and an alkanol, preferably ethanol, in the appropriate proportions. The synthesis takes place at temperatures between 100xc2x0 C. and 200xc2x0 C., with permanent stirring of the gel and a duration between 12 hours and 15 days, preferably between 1 and 7 days. At the end of this time, the reaction product, a white solid with a pH between 11 and 12, is washed with distilled water, filtered and dried.
The laminar material obtained is swollen in a solution which we shall call a swelling solution. For this purpose, the solid is exchanged and/or intercalated with an organic compound with a long chain in order to keep the laminae well separated and thus reduce the attraction forces holding them together. The material used can be any amine or alkyl ammonium compound, preferably cetyl trimethyl ammonium hydroxide (CTMA+). More specifically and, if CTMA+ is used, the conditions for the exchange are: the solutions of CTMA+ (OHxe2x88x92, Brxe2x88x92) (29% in weight) and TPA+(OHxe2x88x92, Brxe2x88x92) (40% in weight) with a proportion of suspension: CTMA:TPA=27:105:33 by weight, are added to a suspension of the laminar material (20% in weight of solid).
It is then left with permanent stirring and reflux, at a temperature between 20xc2x0 C. and 200xc2x0 C., and preferably between 40xc2x0 C. and 120xc2x0 C., for not less than 1 hour, until the material swells.
The material obtained (PREITQ-18) is washed exhaustively with water and dried at temperatures below 300xc2x0 C., and preferably below 150xc2x0 C. Once washed and dried, the swollen material presents a characteristic X-ray diffraction diagram as shown in FIG. 1, the basal spaces and relative intensities of which are summed up on Table 1.
In this description, and unless otherwise specified, the relative intensities of the X-ray diffraction peaks will be represented with the symbols and meanings established below:
w . . . weak . . . 0-20% relative intensity
m . . . medium . . . 20-40%
s . . . strong . . . 40-60%
vs . . . very strong . . . 60-100%
Then, the laminae of the swollen material are dispersed. For this purpose, an aqueous suspension is prepared in a H2O/swollen material weight proportion of between 4 and 200, and preferably between 10 and 100. This suspension is subjected to a controlled stirring process by mechanical means, ultrasounds or any other known means, such as lyophilization, during a period of between 30 minutes and 20 hours, and preferably between 1 minute and 10 hours.
The gelling of the system contributes to increasing the difficulties of filtration to an extraordinary extent. In order to improve these, the addition of flocculants, such as ClH, AcH or NO3H, can be used and/or the subsequent centrifuging of the suspension.
The material obtained, once dry, presents an X-ray diffraction diagram (FIG. 2) where it is evident that, even when maintaining order at a short and long distance, the majority of the diffraction peaks corresponding to the laminar characteristics of these materials have disappeared.
The calcination of this material between 300xc2x0 C. and 800xc2x0 C. and, preferably, between 400xc2x0 C. and 600xc2x0 C., gives rise to the product called ITQ-18. The ITQ-18 material presents an X-ray diffraction diagram such as the one in FIG. 3, with basal spacings and relative intensities summarised on Table 2.
The product obtained (delaminated zeolite ITQ-18) has unique characteristics of specific surface, porosity, acidity, thermal stability and catalytic behaviour. On Table 3, the values obtained by applying the BET equation to the values of the nitrogen absorption isotherm at the temperature of liquid nitrogen are summarised. On this Table, it can be observed that the external surface of the ITQ-18 is much larger than that of any of the zeolitic material described in the bibliography.
If the initial laminar compound is a silicoaluminate, then the delaminated compound has structural acidity (replacement of Si+4 by Al+3) and thus, it will be catalytically active. In addition, on account of the fact that the external surface increases, the accessibility of these cases will increase proportionately. Table 4 shows the values in xcexcmol of pyridine absorbed at different temperatures per gram of sample for the zeolite ITQ-18.
The material called ITQ-18 has a chemical composition represented by the formula (XO2)n(Y2O3)m(H2O)p, where X represents a tetravalent element and Y a trivalent element, with the atomic ratio between X and Y being at least 5.
Preferably, X in XO2 represents, at least, one tetravalent element selected from among silicon, germanium and, most specifically, silicon, which can in some cases contain titanium as well.
Preferably, Y in Y2O represents, at least, one trivalent element selected from among aluminium, boron, iron, chromium and gallium, and most specifically aluminium.
The oxide material of the present invention can comprises in its structure tetravalent, trivalent, bivalent and pentavalent elements. Preferred bivalent elements are Co, Ni, Zn, Mn, and as pentavalent element V is the preferred one.
The oxide material ITQ-18 can be subjected to a post-calcination in the presence of fluorine or a fluorine compound, or a treatment with a phosphorus compound.
The present invention also refers to a product which is the result of the subsequent treatments the ITQ-18 may undergoes according to the preceding paragraph.
The methods of preparation and the catalytic characteristics of the resultant materials are described below by means of non-restrictive examples.